1. Technical Field of the Invention
The present invention relates generally to digital color image sensors, and specifically to a two-color photo-detector capable of detecting two colors at a single photo-detector location and a method for demosaicing a two-color photo-detector array.
2. Description of Related Art
Digital color image sensors are predominately of two types: CCDs (Charge Coupled Devices) and CMOS—APS (Complimentary Metal Oxide Semiconductor—Active Photo-detector Sensors). Both types of sensors typically contain an array of photo-detectors (e.g., pixels), arranged in rows and columns or arranged in other patterns, that sample color within an image. Each photo-detector measures the intensity of light within one or more ranges of wavelengths, corresponding to one or more perceived colors.
In addition, both types of sensors may include a color filter array (CFA), such as the CFA described in U.S. Pat. No. 3,971,065 to Bayer (hereinafter referred to as Bayer), which is hereby incorporated by reference. With the Bayer CFA, each photo-detector sees only one wavelength range, corresponding to the color red, green or blue. To obtain the sensor values for all three primary colors at a single photo-detector, it is necessary to interpolate the color sensor values from adjacent photo-detectors. This process of interpolation is called demosaicing. Demosaiced images frequently exhibit color aliasing artifacts (distortion) due to the inherent under-sampling of color on an image sensor fitted with a CFA. In order to overcome some of the problems associated with color aliasing artifacts, alternative sensor designs have been proposed.
For example, in one alternative sensor design that does not use a CFA, a special prism separates and captures the three primary colors at the same photo-detector location, as is described by Richard F. Lyon in “Prism-Based Color Separation for Professional Digital Photography,” Proceedings of 2000 PICS Conference, IS&T, p. 50-54, which is hereby incorporated by reference. However, the cost of the prism and optics is extremely high. In addition, the need to manually align, in both X and Y, the three sensors and optics to less than a fraction of the width of a photo-detector, which is on the order of 3 microns, is prohibitive for many imager applications.
Another type of sensor design is described in both U.S. Pat. No. 5,998,806 to Stiebig et al. and U.S. Pat. No. 5,965,875 to Merrill, which are both hereby incorporated by reference. The Stiebig et al. and Merrill sensors stack three separate color photodiodes, and electrically connect the photodiodes together to form one photo-detector capable of sensing all three primary colors at a single spatial location. However, both the Stiebig et al. sensor and Merrill sensor include common anodes, such that any current coming out of a three-color photo-detector location is a combination of more than one photodiode current (i.e., the photo-detectors are not electrically isolated from each other). Therefore, in order to measure the differences in current coming out of all three photodiodes, a significant amount of extra circuitry is required, which can be both cost prohibitive and area prohibitive.
A further alternative sensor design is described in an article by K. M. Findlater et al. entitled “Buried Double Junction Photo-detector Using Green and Magenta Filters,” 1999 IEEE Workshop on CCDs and Advanced Image Sensors, pp. 60-64, which is hereby incorporated by reference. Instead of a “three color photo-detector,” as described in Stiebig et al. and Merrill, the Findlater article describes a “two color photo-detector.” In the Findlater sensor, each photo-detector includes two back to back photodiodes resident in the bulk silicon. In addition, a non-Bayer color filter array (CFA) mosaic covers the Findlater sensor. Thus, for every two photo-detectors, four different color values are extracted.
Although the Findlater design provides more accurate color reconstruction as compared to the conventional “Bayer” pattern, the color separation of the two bulk photodiodes is poor, since the absorption spectrum (i.e., sensitivity regions) of each of the sensors is fixed. In addition, with the Findlater design, the photo-detector itself is large due to the fact that two photodiodes and all of the circuitry for each photo-detector are integrated into the bulk silicon, adding both area and cost. What is needed is a new alternative sensor design that samples more than one color at each photo-detector location with improved color separation and reduced area as compared with the Findlater design and electrical isolation between the photo-detectors as compared with previous three-color sensor designs.